Two-dimensional (2D) atomic crystals have emerged as a class of materials that may impact our future electronics technology. A key issue is controlling their electronic state to overcome the limit of natural properties. Black phosphorus is an emergent 2D atomic crystal that is attracting growing interest owing to its promising device characteristics1. In this talk, I will introduce our recent angle-resolved photoemission spectroscopy studies on the widely tunable bandgap of black phosphorus. The in-situ deposition of alkali-metal atoms on the surface of black phosphorus produces strong vertical electric field, modulating the bandgap from a narrow-gap semiconductor to a zero-gap semimetal2. At the critical point of this semiconductor-semimetal transition, black phosphorus is an unusual zero-gap semimetal, whose band dispersion is highly anisotropic, linear in armchair and quadratic in zigzag directions2. I will also briefly discuss about the future direction of this emerging field.